An existing motorcycle has a power unit mounted on the frame structure of the vehicle by means of a swing-arm support structure. With such a support structure, the power unit which consists of an engine and a power transmission mechanism is suspended from the frame structure of the vehicle in such a manner as to form part of a swing-arm structure which is swingable with respect to the frame structure. The swing-arm power-unit support structure includes a suspension assembly which intervenes between the power unit and the rearmost end of a downtube which forms part of the frame structure of the vehicle. The suspension assembly includes a rigid link member pivotally coupled at one end to the power unit and at the other to the downtube with an elastic damper element interposed between the rear end of the downtube and the link member. The link member has upper and lower projections at its end connected to the downtube.
The power unit supported by means of such a suspension assembly is subjected not only to impacts transferred from the road wheels through the frame structure but also to vibrations created in the power unit per se. The impacts transferred through the frame structure cause the power unit to oscillate vertically with respect to the frame structure and the vibrations originating in the power unit tend to be transmitted through the link member to the downtube. The oscillatory motions of the front end portions of the power unit cause the link member to rock vertically with respect to the downtube. Under such conditions, the elastic damper element provided between the downtube and the link member attenuates such oscilatory motions of the power unit and absorbs the vibrations to be transmitted through the link member to the downtube. As a result of the rocking motions of the link member, the elastic damper element is subjected to compressive forces alternately applied from the upper and lower projections of the link member. In this instance, the distance between the axis about which the link member is caused to rock and the area in which the compressive forces are concentrated in the elastic damper elements is limited by the diameter of the downtube as will be discussed in more detail. To enable the elastic damper element to attenuate the oscillating motions of the link member satisfactorily, the elastic damper element is required to provide a relatively large spring constant. Such a damper element is however not adapted to dampen the vibrations originating in the power unit per se and, thus, the vibrations created in the power unit are allowed to transmit through the elastic damper element to the frame structure of the motorcycle in a significant proportion.
It is, accordingly, a prime object of the present invention to provide an improved motorcycle power-unit support structure which is capable of not only attenuating the oscillatory motions of the power unit but absorbing the vibrations which originate in the power unit per se.